Patent GB-1476554-A (GB)
Title: Piezo-optical measuring transducers
IPC: G01L1/24
Filed: 1975-05-16
Abstract: 1476554 Piezo-optical transducers MOSKOV GOSUDARSTVENNY UNIVERSITET IM MV LOMONOSOVA 16 May 1975 21009/75 Heading G1A A Piezo-optical transducer, Fig. 11, comprises a polarizer 84, an elastic element 78 of silicate glass or of a single crystal which varies its optical anisotropy with mechanical stress, a phase-shifting plate 85, an analyzer 86 and silicon photodiodes 87 and 89 electrically connected in a differential relationship, the optical arrangement providing that variations in the intensity of the light falling on the two photodiodes caused by the applied stress are of opposite polarity. The principle of operation is that the element 78 polarizes each light beam in two mutually perpendicular planes and produces a phase difference between the two polarized components due to the difference in refractive index of the element 78 along the two planes. The portions of the polarized components passed by the analyzer 86 interfere and produce a variation in the intensity of the light falling on the photodiodes. This intensity varies sinusoidally with the phase difference Fig. 31 (not shown), and the phase-shifting plate 85 of a birefringent material such as mica is used to bring the working point of the optical channels to the portion of the sinusoid (ac) having maximum slope and linearity. In the embodiment of Fig. 11 the element 78 is subjected to bending stress and the required opposite polarities of the light intensity variations are produced by the two light beams 83, 88 passing on opposite sides of the neutral axis 90. In the embodiment of Fig. 2 the element 4 is subjected to direct compression and the required opposite polarities are produced by having a separate phaseshifting plate 8, 9 for each light beam, the thickness of plate 9 being sufficiently greater than plate 8 to shift the phase difference for beam 15 by an additional amount #. Alternatively the phaseshifting plates may have the same thickness but be arranged so that the axes of maximum and minimum velocities F and S of one plate are normal to respective axes of the other plate, Figs. 3, 4, 5 (not shown). In a still further variation, Figs. 6, 7, 8, 9 (not shown), the plates are of the same thickness and optical alignment but the polarization planes of the polarizer and analyzer are parallel in one channel and perpendicular in the other. For better sensitivity an additional pair of optical channels may be provided (Fig. 9) having additional photodiodes 75, 76, Fig. 14, wherein F.E.T.'s 96, 97 form a differential follower, the gallium arsenide light sources 58-61 are connected in series to power supply terminals 101 and resistors 98-100 serve for zeroing the output signal at zero force. A sensitivity stabilizing circuit Fig. 18 (not shown) comprises a controlled power supply (106) connected to the light source (1) or sources, a Zener diode reference circuit (105), a comparison circuit (104) and adder potentiometer (103). Compensation for temperature drift in the zero output signal is provided by a resistive circuit (118) in which a compensating signal derived from a potentiometer (119) connected in the supply line to the light source is applied to the transducer output terminals (102) via a further potentiometer (120) and resistors (121, 122). Accelerometers Figs. 19-22 (not shown), pressure gauges Figs. 23-25 (not shown), load cells Figs. 26-29 (not shown), and a thermometer Fig. 30 (not shown), embodying the invention are described.